1. Field of the Invention
This invention relates to a heat developable photosensitive material containing an organic silver salt.
2. Description of the Prior Art
Photosensitive materials for forming silver images have been widely used and can give high quality images which can be formed at high sensitivity. As the materials for forming silver images, there are usually known materials using organic silver salts as well as conventional silver halide emulsions. The silver image forming heat developable photosensitive materials using organic silver salts can be developed only by a heat treatment after imagewise exposure. These silver image forming heat developable photosensitive materials can easily form images, in particular, the images can be formed by a dry process, and therefore, these materials have various advantages different from those resulting from conventional silver halide emulsions of a wet developing type and are expected to have wide application fields.
Such heat developable photosensitive materials comprising an organic silver salt contain an organic silver salt and a halide as essential components. As is known (see, e.g., "Imaging Materials and Systems", 1973 Symposium of Society of Photographic Scientists and Engineers, Tokyo, Japan, pages I-4-1 to I-4-9, such salt and halide, when mixed, react immediately to form the corresponding silver halide. The image formation is carried out by imagewise exposure of the heat developable photosensitive material and then heat development. In detail, the imagewise exposure causes the isolation of a small amount of silver resulting in the formation of a latent image, and the small amount of silver thus isolated can be the developing nucleus for silver isolated from the organic silver salt by the subsequent developing procedure to produce silver images at the exposed portions and thus complete the formation of visible images.
These heat developable photosensitive materials comprising an organic silver salt usually do not have a high sensitivity because the materials are not mainly composed of a photosensitive substance of high sensitivity as in the case of conventional silver halide emulsions, and further the original photosensitive composition remains at the non-exposed portion without being subjected to any change and therefore, isolation of silver at the non-exposed portion (non-image portion) is observed upon heat development, and it is very difficult to produce images of high contrast as in the case of a silver halide emulsion. In addition, even after the formation of images (silver images), the non-exposed portions have the same composition as that of the original (before exposure) photosensitive composition, and therefore, isolation of silver from the remaining organic silver salt is observed. Fogging phenomenon is observed after the formation of images. Consequently, it is not possible to maintain the image quality (particularly, contrast) obtained at the image formation, and the image stability is not sufficiently high.